Modern computing and display technologies have facilitated the development of systems for so-called “virtual reality” or “augmented reality” experiences, wherein digitally reproduced images or portions thereof are presented to a user in a manner where they seem to be, or may be perceived as, real. A virtual reality (VR) scenario typically involves presentation of digital or virtual image information without transparency to other actual real-world visual input, whereas an augmented reality (AR) scenario typically involves presentation of digital or virtual image information as an augmentation to visualization of the actual world around the end user.
For example, referring to FIG. 1, an augmented reality scene 4 is depicted wherein a user of an AR technology sees a real-world park-like setting 6 featuring people, trees, buildings in the background, and a concrete platform 8. In addition to these items, the end user of the AR technology also perceives that he “sees” a robot statue 10 standing upon the real-world platform 8, and a cartoon-like avatar character 12 flying by which seems to be a personification of a bumble bee, even though these elements 10, 12 do not exist in the real world. As it turns out, the human visual perception system is very complex, and producing a VR or AR technology that facilitates a comfortable, natural-feeling, rich presentation of virtual image elements amongst other virtual or real-world imagery elements is challenging.
VR and AR systems typically employ head-worn displays (or helmet-mounted displays, or smart glasses) that are at least loosely coupled to a user's head, and thus move when the end user's head moves. If the end user's head motions are detected by the display system, the data being displayed can be updated to take the change in head pose (i.e., the orientation and/or location of user's head) into account.
As an example, if a user wearing a head-worn display views a virtual representation of a three-dimensional (3D) object on the display and walks around the area where the 3D object appears, that 3D object can be re-rendered for each viewpoint, giving the end user the perception that he or she is walking around an object that occupies real space. If the head-worn display is used to present multiple objects within a virtual space (for instance, a rich virtual world), measurements of head pose can be used to re-render the scene to match the end user's dynamically changing head location and orientation and provide an increased sense of immersion in the virtual space.
Head-worn displays that enable AR (i.e., the concurrent viewing of real and virtual elements) can have several different types of configurations. In one such configuration, often referred to as a “video see-through” display, a camera captures elements of a real scene, a computing system superimposes virtual elements onto the captured real scene, and a non-transparent display presents the composite image to the eyes. Another configuration is often referred to as an “optical see-through” display, in which the end user can see through transparent (or semi-transparent) elements in the display system to view directly the light from real objects in the environment. The transparent element, often referred to as a “combiner,” superimposes light from the display over the end user's view of the real world.
VR and AR systems typically employ a display system having a projection subsystem and a display surface positioned in front of the end user's field of view and on which the projection subsystem sequentially projects image frames. In true three-dimensional systems, the depth of the display surface can be controlled at frame rates or sub-frame rates. The projection subsystem may include one or more optical fibers into which light from one or more light sources emit light of different colors in defined patterns, and a scanning device that scans the optical fiber(s) in a predetermined pattern to create the image frames that sequentially displayed to the end user.
In one embodiment, the display system includes one or more planar waveguides that are generally parallel to the field of view of the user, and into which light from the optical fiber(s) is injected. One or more linear diffraction gratings are embedded within the waveguide(s) to change the angle of incident light propagating along the waveguide(s). By changing the angle of light beyond the threshold of total internal reflection (TIR), the light escapes from one or more lateral faces of the waveguide(s). The linear diffraction grating(s) have a low diffraction efficiency, so only a fraction of the light energy is directed out of the waveguide(s), each time the light encounters the linear diffraction grating(s). By outcoupling the light at multiple locations along the grating(s), the exit pupil of the display system is effectively increased. The display system may further comprise one or more collimation elements that collimate light coming from the optical fiber(s), and one or more optical coupling elements that optically couple the collimated light to, or from, an edge of the waveguide(s).
In a typical optical fiber scanning display system, each optical fiber acts as a vibrating cantilever that sweeps through relatively large deflections from a fulcrum in order to scan the light in accordance with a designed scan pattern. However, due to the large deflections of the collimated light, the size of the optical coupling element(s) must be relatively large, thereby increasing the size of the display system. This size of the optical coupling element(s) becomes more problematic in the case of a stacked waveguide architecture, which requires the optical element(s) associated with the waveguides that are more distance from the scanning optical fiber(s) to be larger to accommodate the larger span of the scanned collimated light.
For example, with reference to FIG. 2, one embodiment of a display system 20 comprises one or more light sources 22 that generate image data that is encoded in the form of light that is spatially and/or temporally varying, an optical fiber 24 optically coupled to the light source(s) 22, and a collimation element 26 that collimates the light exiting the distal end of the optical fiber 24. The display system 20 further comprises a piezoelectric element 28 to or in which the optical fiber 24 is mounted as a fixed-free flexible cantilever, and drive electronics 30 electrically coupled to the piezoelectric element 22 to activate electrically stimulate the piezoelectric element 28, thereby causing the distal end of the optical fiber 24 to vibrate in a pre-determined scan pattern that creates deflections 32 about a fulcrum 34.
The display system 20 includes a waveguide apparatus 38 that includes a plurality of planar waveguides 40a-40e that are generally parallel to the field-of-view of the end user, and one or more diffractive optical elements (DOEs) 42a-42e associated with each of the planar waveguides 40. Light originating from the optical fiber 24 propagates along selected ones of the planar waveguides 40 and intersects with the corresponding DOEs 42, causing a portion of the light to exit the face of the waveguide apparatus 38 towards the eyes of the end user that is focused at one or more viewing distances depending on the selected planar waveguide(s) 40.
The display system 20 further comprises optical coupling elements in the form of diffractive optical elements (DOEs) 44a-44e that are integrated within the ends of the respective planar waveguides 40a-40e and that reflect the collimated light into selected ones of the planar waveguides 40. As can be seen, as the distance between each DOE 44 and the end of the optical fiber 24 increases, the length of the respective DOE 44 must increase in order to accommodate the increasing linear span of the deflection angle of the optical fiber 24. This necessarily adds size and complexity to the waveguide apparatus 38 due to the largest DOE 44, and in this case, the DOE 44e. 
As another example, with reference to FIG. 3, another embodiment of a display system 50 is similar to the display system 10 of FIG. 2, with the exception that the display system 50 comprises an optical coupling element in the form of an optical distribution waveguide 52 that have DOEs 54a-54e that reflect the collimate light into selected ones of the planar waveguides 40. The width of the distribution waveguide 52 must be large enough to accommodate the maximum linear span of the deflection angle of the optical fiber 24, thereby necessarily adding size and complexity to the waveguide apparatus 38.
There, thus, is a need to reduce the size of optical coupling element(s) used to couple light from one or more optical fibers into one or more planar waveguides in a virtual reality or augmented reality environment.